1. Field of the Invention
The present invention is directed to a method and apparatus for cooking or heating foods in a microwave oven. More particularly, the invention relates to a microwave cooking vessel adapted for heating two different foods having different microwave absorbing properties.
2. Description of Related Art
In recent years the use of microwave ovens to heat or cook foods has increased markedly, both in the home and in commercial establishments. This statement is true for numerous reasons. For example, microwave ovens require no prewarming, heat efficiently, and result in energy savings. Many foods demonstrate a superior taste when prepared in a microwave oven and retain more of their nutritional components. Microwave ovens are perhaps best known for the speed with which they heat or cook. Microwaves further offer both the homemaker and the commercial establishment rapid reheating of refrigerated pre-cooked foods.
Microwave ovens are not, however, without certain disadvantages. For example, heating or cooking by means of a microwave oven is so rapid that an error of several minutes can make the difference between a well done roast and a rare roast or properly cooked foods and overcooked foods. Each food product, itself, possesses characteristics having a marked influence on cooking or heating time. For example, such a factor as the quantity of the food product to be heated or cooked, the size of the food product, the shape of the food product, its consistency and its dielectric properties all influence the rapidity and uniformity with which it will be heated or cooked in a microwave oven.
Furthermore, microwave ovens by different manufacturers differ in power outputs. Most domestic microwave ovens are produced with power outputs in the range of 600 watts to 1000 watts at a nominal frequency of 2,450 million cycles per second (2,450 MH). The nominal frequency assigned to microwave cooking is 915 MHz with a nominal wavelength of 33 cm (12.9 inches). Finally, the microwaves within the oven chamber tend in some places to reinforce each other and in other places to cancel each other with the result that a food product being heated or cooked in the microwave oven will often demonstrate hot and cold spots, adversely effecting the uniformity of heating or cooking.
All of the above noted factors result in the fact that heating or cooking with a microwave oven is, generally, more critical with respect to time than is conventional heating or cooking. It is not practical to attempt to simultaneously heat or cook several food products having different temperature requirements or energy absorbing characteristics. Furthermore, it is not practical to prepare one set of cooking instructions, based on time, which would be universally applicable to all microwave ovens.
Efforts to overcome the disadvantages of the previous microwave cooking utensils has resulted in numerous designs for packaging and cookware. For example, U.S. Pat. No. 4,416,907 to Watkins relates to a disposable microwave food container having a, generally, bowl-shaped bottom to hold the food. The bottom of the container is transparent to microwave energy. Within the container is a low loss core formed from microwave transparent packaging material. The core extends vertically between the top and bottom of the container to provide a tubular microwave, influx passage through the food within the container. The outer sidewall includes an aluminum foil strip to reflect the microwave energy. In one embodiment, the core extends downward from the top to provide the influx of microwave energy.
U.S. Pat. No. 4,663,506 to Bowen et al. discloses a microwave cake and bread maker for browning a food body. A microwave absorbing material, such as separate layers of ferrite, absorbs microwave energy to provide heat for heating the food by conduction, thermal radiation and convection. The container in one embodiment is formed from a microwave reflective material with a microwave absorbing material on the bottom and side surfaces.
U.S. Pat. No. 4,280,032 to Levinson discloses a cooking container for use in a microwave oven. The container includes an outer microwave permeable container for containing water. An inner container has a coating of a microwave reflective material to limit exposure to the top of an egg within the inner container and to preclude microwave exposure to the bottom and sides. The inner container is nested within the outer container containing the water such that during microwave heating the egg cooks by convection and conductive heat rather than microwave energy.
Other examples of microwave cooking utensils may be found in U.S. Pat. No. 4,144,435 to Clark et al. and U.S. Pat. No. 4,952,765 to Toyosawa. The above-noted inventions have not been entirely successful in producing a microwave cooking vessel. Moreover, the prior art microwave cooking vessels are not able to effectively heat two different food products simultaneously such that the two foods are cooked and heated uniformly.